1. Field of the Invention
The present invention relates to a surface acoustic wave device which improves the propagation velocity (V) and the electromechanical coupling coefficient (K.sup.2) of a surface acoustic wave.
2. Related Background Art
Surface acoustic wave devices using a surface acoustic wave (to be referred to as an "SAW" hereinafter) propagating on a solid surface have the following characteristic features which are common to electromechanical functional parts.
1) Compact and lightweight.
2) Excellent in vibration resistance and high-impact properties.
3) Highly reliable because of few product variations.
4) Since the circuits need no adjustment, the mounting process can be easily automated and simplified.
In addition to the above characteristic features common to electromechanical functional parts, the SAW devices also have various advantages such as a relatively good temperature stability, a long service life, and excellent phase characteristics. For this reason, the SAW devices can be popularly used as frequency filters, resonators, delay devices, signal processing devices, convolvers, opto-electronic functional devices, and the like.
As is known, for such SAW devices, a multilayer structure with an LiNbO.sub.3 layer formed on diamond is used, paying attention to the fact that LiNbO.sub.3 is chemically stabler (acid resistance and alkali resistance) than, e.g., ZnO.
For application as a frequency filter, an electromechanical coupling coefficient (K.sup.2) used as an index of conversion efficiency from an electrical energy to a mechanical energy is about 0.15% to 0.7% for a narrowband filter, about 0.7% to 3% for an intermediate-band filter, or 3% to 10% for a wideband filter.
In the field of the above-described SAW devices, along with a recent tendency in multichannel or higher-frequency arrangements in the field of communications including satellite communication and mobile communication, the development of devices usable in a higher frequency range (e.g., GHz band) has been desired.
An operating frequency f of an SAW device is generally determined by f=V/.lambda. (V is the propagation velocity of an SAW, and .lambda. is the wavelength of the SAW). The wavelength .lambda. depends on the period of an interdigital transducer, as will be described later. However, the wavelength .lambda. of an SAW to be used for the device can hardly be extremely shortened because of the limitation in micropatterning technique such as photolithography. Therefore, to raise the operating frequency of the SAW device, it is preferable to increase the propagation velocity V of the SAW.
In the above-described field of communications represented by satellite communication and mobile communication, further power saving and size reduction of an entire device are required mainly from the viewpoint of mounting of the SAW device. In addition to the above-described higher frequency, an improvement in the electromechanical coupling coefficient (K.sup.2) as the index of conversion efficiency from an electrical energy to a mechanical energy is required.
In recent years, therefore, for SAW devices to be widely used, a strong demand for not only an increase in propagation velocity V of an SAW to be used for the devices (e.g., V.gtoreq.7,000 m/s) but also an increase in electromechanical coupling coefficient (K.sup.2) (e.g., K.sup.2 .gtoreq.2%) has arisen.